Detectors of this type use ionization chambers like those described for example in French Pat. No. 2,292,985. These ionization chambers are constituted by a tight enclosure provided with a window permeable to the beam of ionizing radiation (X or Y rays). The enclosure contains metal plates or electrodes which are substantially parallel to one another and perpendicular to the window. These electrodes are raised to potentials of given values, so as to establish a high electrical field (several thousand Volts/cm) and which is also as uniform as possible between two successive electrodes. A gas with a high atomic number is introduced at high pressure into the tight enclosure in such a way that the beam of ionizing radiation entering the enclosure ionizes the gas which it contains, thus freeing the ions and electrons which are respectively collected by the electrodes.
However, in ionization chambers, electrical field lines generally have deformations at the end of the electrodes. These deformations are due to the projection of the electrical field at the ends of the electrodes and to the presence of the intake window located in the vicinity thereof, as described in French Pat. No. 2,348,567.
Therefore, part of the electrical charges is not collected by the electrodes, which reduces the efficiency of the detector and can also lead to stray currents. The electrical fields in fact undergo deformations such that the ions and/or electrons produced in the space between the collecting electrodes and the window cannot be collected by these electrodes and consequently do not contribute to the electrical signals supplied at the detector output.
In order to obviate these disadvantages it has been proposed (French Pat. No. 2,348,567) to place a layer of dielectric material on the inner surface of the window. The disadvantage of this solution is to eliminate the autocollimation effect of the detector, which is particularly necessary for eliminating diffused radiation.
It is therefore necessary for the detector to be equipped with a device which serves both as a collimator and a guard electrode, whilst in no way prejudicing the efficiency of the detector. One solution is to arrange a guard electrode in the extension of each of the measuring electrodes in the ionization chamber, the guard electrode being at the same potential as the electrode which it extends, thereby eliminating the deformations of the electrical field. However, if the guard electrodes are located in the same enclosure, they would collect charged particles (ions or electrons) which reduces the efficiency of the detector. However, if these guard electrodes are positioned outside the ionization chamber, the distance separating the guard electrode and the corresponding measuring electrode will be considerable, due to the thickness of the window, which has to withstand a considerable pressure difference, which will bring about considerable overlapping of the electrical field.
The detector according to the invention eliminates these disadvantages.